1. Field of the Invention
The present invention relates generally to electrical power conversion, and more particularly, to sensing and monitoring of electrical power in a DC-to-DC switching-mode power converter.
2. Description of the Related Art
To convert one DC (Direct Current) level to another, a DC-to-DC switching-mode converter is commonly employed to perform the task. FIG. 1 shows a conventional DC-to-DC switching-mode converter signified by the reference numeral 2. The converter 2 has an input circuit 4 and an output circuit 6 separated by a transformer 8. The input circuit 4 includes a switch 10 controlled by a control circuit 12. One terminal of the switch 10 is tied to the primary winding 12 of the transformer 8. The other terminal of the switch 10 is connected to the DC input VIN. The output circuit 6 includes an inductor 15 and a capacitor 16 connected in series. The common connection of the inductor 15 and the capacitor 16 drives a load 18. The primary and secondary windings 12 and 20 of the transformer 8 have N1 and N2 winding turns, respectively. Disposed between the secondary winding 20 of the transformer 8 and the inductor 15 is a diode 14. Further, connected across the inductor 15 and the capacitor 16 combination is another diode 17.
During operation, an input DC voltage VIN is supplied to the one terminal of the switch 10. The control circuit 12 generates a periodic output which in essence periodically turns on and off the switch 10. As a consequence, a time-varying current iP with periodic current pulses flows through the primary winding 12 of the transformer 8. In this specification, the lower case alphabets are used to designate parameters that vary with time. Since the primary and secondary windings 12 and 20 are inductively coupled together, a secondary current iS is thereby induced in the secondary winding 20. The secondary current iS passes through the diode 14 which admits only positive current cycles but blocks away any negative counterparts. Since both the inductor 15 and the capacitor 16 respectively assume high inductive and capacitive values, they cooperatively contribute to a slow decaying time-constant. The secondary current iS with only positive current cycles, after passing through the diode 14, charges and decays sluggishly through the inductor 15 and the capacitor 16. The installation of the diode 17 provides a free-wheel current path when the switch 10 is turned off. As a consequence, a DC voltage level is basically maintained across the capacitor 16. The DC voltage level is utilized as the DC output voltage VOUT driving the load 18. Depending on the impedance of the load 18, a DC current IOUT is established passing through the load 18, in accordance with Ohm""s law.
In practice, the load current IOUT needs to be monitored. Insufficient current flowing through the load 18 may render the load 18 inoperative or malfunctional. On the other hand, excessive current IOUT feeding the load 18 may damage the load 18 and also the power converter 2. Different applications require different current monitoring schemes. For example, in some applications in which the load 18 may require over current protection and thus the upper limit of the output current IOUT must be detected and maintained. As another example, in a shared-load arrangement, the common current IOUT driving the shared load 18 needs also be ascertained for proper load current allocation. Furthermore, in usages where the instantaneous power needs to be known, the instantaneous value of the output current IOUT must also be instantaneously detected and reported.
Heretofore, monitoring of the output current IOUT has mostly been conducted on the secondary side of the transformer 8 by directly measuring the current path through the load 18. A common approach is to place a shunt resistor in series with the load 18. Another known approach is to couple a Hall effect device to the load 18.
First, the use of a Hall effect device involves complicated circuit design and thus costly. In addition, a Hall effect device is spacious. The use of Hall effect devices in most instances are not practical.
The use of shunt resistors for current detection is a common practice but it involves considerable shortfalls. To understand the drawbacks associated with using a shunt resistor, the basic principles of a DC-to-DC converter needs first be explained. Reference is now directed back to FIG. 1. In the DC-to-DC converter 2, if the transformer 8 is a step-down transformer, as is known in the art, the primary and secondary voltages vP and vS, across the primary and secondary windings 12 and 20, respectively, assume a directly proportional relationship in accordance with the following algebraic expression:                                           v            P                                v            S                          =                  N1          N2                                    (        1        )            
However, the primary and secondary currents iP and iS relate to each other by an inversely proportional relationship as expressed by the following mathematical relationship:                                           i            P                                i            S                          =                  N2          N1                                    (        2        )            
In a step-down transformer, the secondary voltage vS is lower than the primary voltage vP. However, the secondary current iS is higher than the corresponding primary current iP. In most applications with a DC-to-DC converter, such as the converter 2, the output voltage VOUT is much lower than that input voltage VIN, resulting in the output current IOUT much higher than the corresponding input current Iin. In practice, sensing a high current always posses technical complications and sometimes fraught with danger. Chief among all is the difficulty in the power management of the shunt resistor. Even though the shunt resistor is normally designed to have a small ohmic value, in terms of degree of difficulty in managing the power of the shunt resistor, the high output current IOUT passing through the shunt resistor more than compensates for the choice of low resistive value of the shunt resistor in the first place. As is well known, power consumption of a resistor when current passes through the resistor has the following relationship:
P=IOUT2Rxe2x80x83xe2x80x83(3)
where P is the power consumed by the shunt resistor in Watts; R is the ohmic value of the shunt resistor; and IOUT is as defined above.
Very often, to make up for the lowering of the resistive value R of the shunt resistor, a shunt resistor with a large physical size has to be selected. Modern day designs of power converters require compactness where the use of large components are not practical. The shunt resistor usually needs to be scaled down in physical size. As a consequence, current through the shunt resistor must be increased resulting in excessive power loss via wasteful heat generated out of the shunt resistor. As shown in equation (3), the relationship between the power consumption P and the current IOUT is not linear, Rather, the power consumption P is proportional the square of the current IOUT passing through the resistor. A small increase in current always results in a significant increase in power dissipation. Further, as is also known in the art, heat also effects the resistive value of a resistor. Excessive self-generated heat from the shunt resistor may render the resistive value of the shunt resistor unreliable and thus may yield inaccurate current reading of the output current IOUT. Sophisticated thermal management or temperature compensation circuitry may be implemented to rectify such shortfalls but it surely will result in high manufacturing cost and design complication.
Without resorting to costly and complex designs, there is a need to provide better solutions in sensing output current of a DC-to-DC power converter.
It is accordingly the object of the invention to provide a DC-to-DC power converter with a current sensing mechanism having relative ease and simplicity in implementation. It is also another object of the invention to provide such a converter at low cost and high operational reliability.
The DC-to-DC power converter in accordance with the invention includes a transformer disposed between an input circuit and an output circuit. The transformer has primary and secondary windings coupled to the respective input and output circuits. Current passing through the input circuit is sensed and detected by a detecting circuit which generates a signal proportional in magnitude to the output current sourcing out of the output circuit. If the converter is a step-down converter, the output current is higher than the input current. As arranged, sensing and monitoring input current instead of the output current allows simpler circuit design, lower cost and higher operational reliability.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which like reference numerals refer to like parts.